1. Field of the Invention
The present invention relates to a fabricating method of a liquid crystal display (LCD) device, and more particularly to an etching process of a substrate for reducing weight of an LCD device.
2. Discussion of the Related Art
Flat panel display (FPD) devices having small size, lightweight, and low power consumption have been a subject of recent research in the coming of the information age. Among many kinds of FPD devices, LCD devices have been widely developed and used because of their excellent characteristics of resolution, color display and display quality.
Generally, LCD devices include an upper substrate and a lower substrate facing each other with liquid crystal molecules interposed therebetween. Each substrate has an electrode on an inner surface thereof. An electric field is generated by applying a voltage to the electrodes, thereby driving the liquid crystal molecules to display images depending on light transmittance.
The LCD device is fabricated by the processes of forming a lower substrate, referred to as an array substrate, having thin film transistors (TFTs) and pixel electrodes; forming an upper substrate, referred to as a color filter substrate, having common electrodes and color filters; forming a liquid crystal cell by aligning and attaching the substrates; injecting the liquid crystal between the substrates; and sealing and attaching a polarization film. In a conventional LCD device, since a plurality of liquid crystal cells are simultaneously formed on a wide area substrate, a process for cutting the substrate into the unit liquid crystal cells is needed after an assembly process.
FIG. 1 is a flow chart illustrating a fabricating process of a conventional liquid crystal cell.
At step ST1, the lower and upper substrates that include TFTs and color filters, respectively, are formed. The lower substrate is formed by repeating deposition and patterning steps of a thin film and several masks. Recently, a fabrication process that reduces cost by decreasing the number of masks has been investigated. The upper substrate is formed by subsequently making a black matrix, red (R)/green (G)/blue (B) color filters and a common electrode. The black matrix distinguishes the color filters and prevents light leakage of a non-pixel area. The color filter can be formed by a dyeing method, a printing method, a pigment dispersion method or an electro-deposition method; the pigment dispersion method is most widely employed.
At step ST2, an orientation film that determines an initial orientation of the liquid crystal layer is formed on the upper and lower substrates. This step includes deposition and alignment of a polymeric thin film along a specific direction. An organic material of the polyimide series is mainly used as the orientation film and a rubbing method is mainly used as the aligning method of the orientation film. The rubbing method consists of rubbing the orientation film along the specific direction with a rubbing cloth, and has advantages such as easy orientation treatment, suitability to mass production, high stability of the orientation and easy controllability of a pre-tilt angle.
At step ST3, a seal pattern that forms a gap for liquid crystal material injection and prevents leakage of the liquid crystal material is formed on one substrate. The seal patterning process involves forming a desired pattern by application of a thermosetting plastic. A screen print method using a screen mask and a seal dispenser method using a dispenser are used for the seal patterning process. For the simplicity of fabrication, the screen print method has mainly been used. However, since the screen mask is not suitable for a wide substrate and a contamination by contact between the mask and the orientation film often occurs, use of the seal dispenser method has gradually increased.
At step ST4, a spacer having a specific size to maintain a precise and uniform gap between the upper and lower substrates is sprayed onto one of the upper and lower substrates. The spacer spray method can be divided into two different types: a wet spray method that involves spraying a mixture of alcohol and spacer material, and a dry spray method that involves spraying spacer material alone. Furthermore, the dry spray method can be subdivided into two different types: an electrostatic spray method that uses electrostatic force and a non-electric spray method that uses gas pressure. Since the liquid crystal cell structure is susceptible to damage from static electricity, the non-electric method is mainly used.
At step ST5, the upper and lower substrates are attached by pressurized hardening of the seal pattern.
At step ST6, the attached substrates are divided into unit cells. A cell cutting process includes a scribe process that forms cutting lines on a surface of the substrate using a diamond pen, a hardness of which is higher than that of the glass substrate, and a break process that divides the attached substrates into the unit cells by force.
At step ST7, a liquid crystal material is injected into the unit cell. A vacuum injection method using pressure difference between the inside and outside of the unit cells is commonly used as an effective injection method. Since fine air bubbles included in the liquid crystal material can deteriorate the display property of the unit cell, a bubble-eliminating process, in which the unit cells are kept in a vacuum state for a long period of time, is required.
After finishing the liquid crystal material injection, an injection hole is sealed to prevent leakage of the liquid crystal material. Generally, a ultra violet (UV) curable resin is deposited into the injection hole by use of a dispenser and then ultra violet light is irradiated on the resin, thereby hardening the resin and sealing the injection hole. Polarization films are attached on outer surfaces of the unit cell and a driving circuit is connected to the unit cell using an attachment process. After the attachment process, a substrate etching process, in which the outer surfaces of the upper and lower substrates are etched to reduce the thickness of the substrates, is performed according to the desired lightening of the substrate.
FIG. 2 is a flow chart illustrating a substrate etching process.
At step ST11, impurities made during the previous processes are eliminated. If there are impurities on the outer surfaces of the attached substrates, etching quality is deteriorated, such as an under-etching in the vicinity of the impurities occurs and the surfaces of the substrates become rough. Accordingly, since a diffused reflection or a refraction of the light can occur, the impurities are eliminated with a cleaning solution such as isopropyl alcohol (IPA) or deionized water (DI).
At step ST12, the attached substrates are etched. Generally, a glass substrate is used for the liquid crystal substrate and about 60% of the substrate comprises silicon dioxide (SiO2). Therefore, the substrate can be etched with a solution of hydrofluoric (HF) acid which is an etching solution for SiO2.
At step ST13 and ST14, a residue of the HF solution is removed and the substrates are dried.
After etching outer surfaces of the attached substrates, a cutting process ST6 (of FIG. 1) and a liquid crystal injection process ST7 (of FIG. 1) are performed.
However, a seal pattern can be damaged during the substrate etching process. Recently, a forming method of a dual seal pattern is suggested to prevent a damage of a seal pattern resulting from an etching solution permeating through two substrates during the substrate etching process. In the forming method of a dual seal pattern, unit cells are formed on a substrate. Main seal patterns are formed at corresponding unit cells and an auxiliary seal pattern surrounding the main seal patterns is formed between a boundary of the substrate and main seal patterns. Each main seal pattern has an injection hole to inject liquid crystal material. The auxiliary seal pattern prevents a damage of the main seal patterns resulting from an etching solution of a substrate etching process. However, since the auxiliary seal pattern has an open portion to exhaust airs between the attached substrates, the etching solution can permeate into an interior of the unit cells through the open portion. Accordingly, corrosion of a pad of an electrode or breaking of the substrate can occur.